1. Field of the Invention
The present invention relates to a method and apparatus for acquiring and processing projection images of an object of interest, particularly angiographic images.
2. State of the Art
During clinical interventions it is important to obtain as much object information as possible to be able to accurately perform a procedure. For this reason the imaging methods usually adopted are those based on the use of apparatus acquiring high resolution volumetric images in order to have a better and more detailed picture of the object under examination.
Such apparatus, like CT and MR machines, besides having a large size and thus being cumbersome and expensive, provide a great amount of image data, whose processing is time-consuming and makes it impossible to perform imaging sessions in real-time. This drawback largely prevents the use of these machines during interventions.
For example in the field of Transcatheter Aortic Valve Implantation (TAVI), an intervention aiming at planning and placing an aortic valve implant, the gold standard imaging technique for selecting the type of valve and its relative positioning is a Multislice CT due to its high spatial resolution. However, such technique can be used only when planning the operation, for example in order to define the size of the valve, since it can be performed neither in real-time nor in a hemodynamic and/or heart surgery room (so called cathlab) during the intervention.
For this reason such type of interventions are generally performed under guidance of two-dimensional images acquired, for example, with angiographic X-ray systems of the so-called C-arm or L-arm type. These systems are used to acquire two-dimensional images, also called two dimensional projections or projection images, of the object under examination. Several perspectives can be obtained rotating the arm holding the X-ray source and detector with reference to the patient.
However, two-dimensional projection images, as those obtained with angiographic systems, suffer from the problem of foreshortening. Foreshortening is the event when an object seems compressed when viewed from a certain perspective, causing distortion of the information. This is particularly critical when, based upon such information, a clinical intervention is planned and/or performed, such as, for example, the placing of a stent in an artery or vessel in general or a valve implant. When using a two-dimensional imaging modality it is therefore important to acquire images from the right perspective.
The views in which an object of interest are visualized with minimum foreshortening are called optimal views as taught by the paper “Determination of optimal angiographic viewing angles: basic principles and evaluation study”, Adrie C. M. Dumay, Johan H. C. Reiber, Jan J. Gerbrands, IEEE Trans. Med. Imaging, vol. 13, N. 1, March 1994.
In case of angiographic systems, the correct perspective is defined as the angulations of an X-ray system (both the system rotation and angulation) that contains as much information as possible needed for that procedure. This normally happens when the imaging system is positioned in a plane parallel to the main axis of the object i.e. when the projection is perpendicular to the object.
The most present developments in the field focus on one or multiple optimal projections that can be used during clinical interventions. These optimal projections are determined based solely on foreshortening. See for example Joel A. Garcia et al “Determination of optimal viewing regions for X-ray coronary angiography based on a quantitative analysis of 3D reconstructed models” International Journal of Cardiovascular Imaging, 2009, Volume 25, Number 5, Pages 455-462.
A large drawback of this approach is that it assumes that every optimal projection contains the same amount of information of the imaged object (that is that the imaged object is symmetrical). This leads to a variety of possible optimal projections that do not necessarily contain all the object information present in the images. This results in the potential usage of optimal projections that are only suitable for symmetrical objects.
However, as in clinical practice the objects that are dealt with are asymmetrical (see FIG. 1), a recording that is perpendicular to the object does not necessarily contain all the needed information. It would thus be desirable to determine the optimal viewing angle or optimal projection that not only minimizes foreshortening, but also contains all relevant information of the asymmetrical object or device for that clinical intervention.
As the existing developments in the field only focus on the foreshortening aspect, it is presently necessary for a clinician to obtain such an optimal viewing angle by trial and error during an intervention. This procedure is time consuming and is a burden for the patient because several acquisitions, whether or not supported by administration of a contrast agent, have to be made before the desired optimal view has been found. How many acquisitions are needed depends on the experience of the clinician and the patient anatomy.
There's thus a need for a method that would help the clinician to choose the correct perspective from which a three-dimensional object is to be optimally viewed not only in terms of reduction of foreshortening, but also in terms of completeness of shown information. Such information varies from case to case and thus cannot be fixed in advance in each imaging procedure. It is something related to the specific scope an image is taken for.
It is assumed clear that it is the objective of every imaging session to obtain and use as much image information as possible and to restrict the amount of information loss especially with asymmetrical objects. Not every procedure however classifies the same image information as important for the current procedure.
For this reason the prior art is mainly aimed at finding optimal projections reducing the foreshortening problem leaving to the expertise of the clinician the job of finding optimal projections for specific applications with trial and error procedures.